In the oil and gas exploration and production industry, many oil and gas deposits are found in offshore locations. This presents significant challenges, not least of which is the transportation of large volumes of oil and/or gas back onshore. One way in which this is achieved is by laying a pipeline from the offshore location back to an onshore installation. This usually involves laying a pipeline on the seabed, extending from a production and/or storage facility in the offshore location, back onshore. This is, however, an extremely costly and time-consuming procedure.
Advanced pipelaying techniques have therefore been developed which have sought to reduce both the time and cost involved in laying pipelines of this type. These techniques involve manufacturing a length of pipeline of the order of several hundred or evens thousands of meters in length onshore, and then deploying the length of pipeline on to a large diameter reel, which is supported on a dedicated pipelaying vessel. The reeled length of pipeline is then transported offshore and deployed from the reel on the vessel and on to the seabed. The vessel progresses in a forward direction as the length of pipeline is slowly deployed overboard. A number of such reeled lengths of pipeline may be required in order to form the complete pipeline, which may be several thousand meters in length, the successive reeled lengths of pipeline being welded together end-to-end.
Many different techniques have been developed for deploying the pipeline from the vessel, including ‘S-lay’ and ‘J-lay’ techniques, which are well known in the industry. The primary differences between the various techniques are the particular way in which the length of pipeline is stored on the vessel, and the way in which a free end of the pipeline is deployed from the vessel and down towards the seabed.
In each case however, the length of pipeline is deployed from the vessel into open water, and flooded with seawater so as to negate buoyancy effects. The length of pipeline experiences significant stresses as it travels down to the seabed. In particular, it experiences significant bending stresses as it is curved to transition from the angle at which it is deployed from the vessel, to that which it adopts when it is laid on the seabed. In this regard, it will be noted that some pipelines, depending on their purpose and the materials from which they are constructed, can be arranged so that they are raised up from the seabed in certain areas, for example in a ‘lazy-wave’, which allows for heave of a floating production/storage vessel to which the pipeline is connected.
A particular problem which can be encountered during deployment of the length of pipeline is a situation known as a ‘wet buckle’. This is where the pipeline buckles during deployment from the vessel, at a location which is below sea-level. The buckle typically occurs in a region of the pipeline which is undergoing significant curvature, and so consequent stress. When a wet buckle occurs, it is necessary to stop forward motion of the pipelaying vessel, before ‘backing-up’ the vessel and reeling the pipeline back on to the reel. This process continues until the buckle has been brought back to the level of the vessel deck. The pipeline is then securely gripped above and below the buckle, cuts made either side of the buckle, and the buckled portion removed. The cut ends of the pipeline are then subjected to a suitable preparatory procedure and welded together. Pipelaying can then recommence.
In theory this is a simple procedure, but in practice it presents numerous technical challenges, and also hampers subsequent completion of the pipeline.
In particular, as mentioned above, the pipeline is flooded with seawater during deployment. Depending upon water depth, it may not be possible to raise the buckle back up to the vessel, for the simple reason that the self-weight of the pipeline containing the water is too great for equipment on the vessel, and potentially the vessel itself, to support.
More significantly, it is very important that the inside of the pipeline does not become contaminated during the procedure to cut out the buckled section of pipeline, for example with abrasive metal cuttings. A particular reason for this is that most metal pipelines are of a simple low-cost carbon steel, with a thin inner liner of an expensive corrosion resistant material (such as 316 or 317 stainless steel). Metal cuttings within the pipeline could lead to a deterioration of the liner and consequent corrosion of the carbon steel, with potentially disastrous consequences. It is therefore necessary to provide a barrier within the pipeline before commencing the cutting procedure.
All pipelines which are deployed following the above techniques include a ‘laydown head’ at their free end, which carries a number of specialist pipe inspection/cleaning devices known as ‘pipeline pigs’. The pigs can be translated along the pipeline, their primary purpose being performance of pressure testing/inspection of the completed pipeline. A number of such pigs are provided in the laydown head, and are separately deployable so that numerous such procedures can be carried out.
When a wet buckle occurs, it is necessary to release a pig from the laydown head, and to pump it along the pipeline to a position which is adjacent the buckle. In shallow water, the pig can be pumped along the pipeline using seawater, but in deep water locations, it is necessary to pump the pig along the pipeline using air, due to the self-weight issues discussed above. This brings about its own problems because it can result in the pipeline becoming buoyant and lifting off the seabed.
Once the pig has been positioned adjacent the buckle, the pipeline can be drawn back up to the vessel, and the buckled portion cut out, as described above. The pig can then be pumped up and out of the pipeline, carrying any contamination such as metal cuttings. The primary problems with this are as follows.
Firstly, it is time-consuming to pump the pig along the pipeline from the laydown head, which may amount to a distance of many kilometers. The entire procedure can take of the order of 14 days to complete. This represents a significant delay in the context of completing the pipeline laying procedure.
Secondly, the procedure can be extremely expensive. This is because pipelaying is delayed, and the specialist pipelaying vessels are hired out at rates of the order of hundreds of thousands of US dollars per day, which are wasted when the vessel is not being employed in pipelaying. Also, in deep-water locations where air must be charged into the pipeline to pump the pig along the pipe, this requires a further specialist vessel which is capable of pumping large volumes of air hundreds of meters below sea level to the laydown head, and also often requires deployment of an ROV to connect supply hoses and actuate the pig. Such equipment is again hired out at significant day rates.
Thirdly, the procedure effectively uses up one of the pipeline pigs which has been positioned in the laydown head prior to deployment of the pipeline from the vessel. This can hamper test/inspection procedures to be carried out at a later date, particularly if more than one wet buckle occurs.